1. Field of the Invention
The present invention relates to a device for estimating a road friction state, and in particular, to a device for estimating a road friction state in which the friction state of a wheel with respect to a road surface is estimated on the basis of a physical amount expressing the ease of slippage of the wheel, such as the road xcexc slope which is the slope of the friction coefficient between the wheel and the road surface, with respect to the slip speed.
2. Description of the Related Art
Conventionally, there has been proposed a device in which a wheel speed signal is detected, and the road xcexc slope is estimated on the basis of the detected wheel speed signal. In this device, the wheel speed signal is detected in order to estimate the road xcexc slope. Thus, the estimated value of the road xcexc slope is small at times when the wheel rides over projections or on rough road surfaces such as on packed snow. Accordingly, the accuracy of estimating the road xcexc slope is poor.
For example, as illustrated in FIG. 10, when the wheel rides over a projection while travelling on an asphalt road, as described above, the wheel speed signal is small. The estimated value of the road xcexc slope estimated at this time is small, and is about the same value as the estimated value of the road xcexc slope on a low xcexc road. When the wheel rides over a projection, there may be cases in which it is judged that the vehicle is traveling on a low xcexc road.
The present invention was developed in consideration of the above facts, and an object thereof is to provide a device for estimating a road friction state which can accurately estimate the friction state of a wheel with respect to a road surface.
A first aspect of the present invention is a device, for use with a vehicle having wheels, for estimating a road friction state, said device comprising: a sensor which detects a wheel speed signal; a filter which receives the signal and passes a portion of the wheel speed signal having a frequency within a predetermined range, the range including at least one resonance point or at least one antiresonance point; a computer which computes a physical amount expressing a magnitude of vibration of a wheel, on the basis of the portion of the wheel speed signal passed by the filter; an estimator which estimates a physical amount expressing ease of slippage of the wheel, on the basis of the portion of the wheel speed signal passed by the filter; and a road friction state estimator which estimates a friction state of the wheel with respect to a road surface, on the basis of the physical amount expressing the magnitude of the vibration of the wheel computed by the computer and the physical amount expressing the ease of slippage of the wheel estimated by the estimator.
The sensor of the present invention detects a wheel speed signal. The filter passes, from the wheel speed signal detected by the sensor, a portion of the wheel speed signal within a predetermined range which includes at least one resonance point or at least one antiresonance point and in which the frequency is great than a low frequency region.
The computer computes a physical amount expressing the magnitude of the vibration of the wheel, on the basis of the portion of the wheel speed signal extracted by the filter. The physical amount expressing the magnitude of the vibration of wheel may be the vibration level or the amplitude of the portion of the wheel speed signal extracted by the filter.
The estimator estimates a physical amount expressing the ease of slippage of the wheel, on the basis of the wheel speed signal detected by the sensor. Here, the physical amount expressing the ease of slippage of the wheel may be the road xcexc slope which is the slope of the friction coefficient xcexc between the wheel and the road surface for a slip speed, or may be the braking torque slope which is the slope of the braking torque for a slip speed, or may be the driving torque slope which is the slope of the driving torque.
The relationship between the physical amount expressing the magnitude of the vibration of the wheel and the physical amount expressing the ease of slippage of the wheel is determined for each road surface state.
Thus, the estimator estimates the friction state of the wheel with respect to the road surface, on the basis of the physical amount expressing the magnitude of the vibration of the wheel computed by the computer and on the basis of the physical amount expressing the ease of slippage of the wheel estimated by the estimator.
The friction state of the wheel with respect to the road surface is estimated on the basis of the physical amount expressing the magnitude of the vibration of the wheel and the physical amount expressing the ease of slippage of the wheel. Thus, the friction state of the wheel with respect to the road surface can be judged accurately.
The physical amount expressing the magnitude of the vibration of the wheel increases proportionally to the reciprocal of the physical amount expressing the ease of slippage of the wheel. Namely, the physical amount expressing the magnitude of the vibration of the wheel and the physical amount expressing the ease of slippage of the wheel exhibit an inverse relationship. As a result, even if the friction state of the wheel with respect to the road surface is estimated on the basis of the physical amount expressing the magnitude of the vibration the wheel and the physical amount expressing the ease of slippage of the wheel which are determined by extracting a portion of the wheel speed signal in a low frequency region, the friction state of the wheel with respect to the road surface cannot be judged accurately.
Here, in the present invention, in order to estimate the friction state, a portion of the wheel speed signal in a predetermined range which is greater than a low frequency region is extracted, and the friction state of the wheel with respect to the road surface can be judged accurately.
Another aspect of the present invention is a device, for use with a vehicle having wheels, for estimating a road friction state, said device comprising: a sensor which detects a wheel speed signal; an estimator which estimates a physical amount expressing an ease of slippage of a wheel, on the basis of the wheel speed signal detected by the sensor; a computer which computes a rate of change in the physical amount expressing the ease of slippage of the wheel which was estimated by the estimator; and a road friction state estimator which, on the basis of the physical amount expressing the ease of slippage of the wheel estimated by the estimator and the rate of change in the physical amount expressing the ease of slippage of the wheel computed by the computer, estimates a friction state of the wheel with respect to a road surface.
The relationship between the physical amount expressing the ease of slippage of the wheel and the rate of change in the physical amount expressing the ease of slippage of the wheel is determined for each road surface state.
The road friction state estimator estimates the friction state of the wheel with respect to the road surface on the basis of the physical amount expressing the ease of slippage of the wheel estimated by the estimator and the rate of change in the physical amount expressing the ease of slippage of the wheel which is computed by the computer.
In this way, the friction state of the wheel with respect to the road surface is estimated on the basis of the physical amount expressing the ease of slippage of the wheel and the rate of change in the physical amount expressing the ease of slippage of the wheel. Thus, the friction state of the wheel with respect to the road surface can be judged accurately.
In the present invention, in order to estimate the friction state, a portion of the wheel speed signal in a predetermined range which is greater than low frequency region is extracted. Thus, the friction state of the wheel with respect to the road surface can be judged accurately.